1. Field of the Invention
The present invention relates to a vehicle navigator capable of detecting the position of a vehicle.
2. Description of the Related Art
A conventional vehicle navigator will be described.
FIG. 2 is a functional block diagram of a conventional vehicle navigator. In FIG. 2, reference numeral 1 represents an acceleration sensor for detecting an acceleration of a vehicle along its running direction to calculate the running distance of the vehicle. Reference numeral 2 represents a gyro sensor for detecting an angular velocity of the vehicle along its yawing direction to calculate the running azimuth of the vehicle. Reference numeral 3 represents a global positioning system (GPS) receiver for receiving radio waves from a plurality of GPS satellites to obtain positioning data of the vehicle. Reference numeral 4 represents a running distance calculator for calculating the running distance of the vehicle in accordance with output data of the acceleration sensor 1. Reference numeral 5 represents a running azimuth calculator for calculating the running azimuth of the vehicle in accordance with output data of the gyro sensor 2. Reference numeral 6 represents a relative position calculator for calculating the present position of the vehicle by adding the vehicle running distance and the vehicle running azimuth output from the running distance calculator 4 and the running azimuth calculator 5, to the previous vehicle position. Reference numeral 7 represents a reliability judging unit for judging the reliability of positioning data output from the GPS receiver 3. Reference numeral 8 represents a velocity correcting unit for correcting a velocity of the vehicle calculated from data output from the acceleration sensor 1. Reference numeral 9 represents a position determining unit for determining the position of the vehicle on the basis of the position of the vehicle output from the GPS receiver 3 and the relative position calculator 6. Reference numeral 10 represents a map storage for storing map data. Reference numeral 11 represents a display unit for displaying image data such as the position of the vehicle or map data. Reference numeral 12 represents a controller for fetching from the map storage 10 map data of a predetermined area corresponding to the position of the vehicle on the basis of the data output from the position determining unit 9, and outputting the vehicle position and map data to the display unit 11.
FIG. 3 is a conceptual diagram showing a conventional vehicle navigator. As shown in FIG. 3, the acceleration sensor 1 and the gyro sensor 2 are mounted on a vehicle 13 so that they can detect an acceleration of the vehicle along its running direction and an angular velocity of the vehicle along its yawing direction, respectively. The GPS receiver 3 is also mounted on the vehicle. An antenna (not shown) of the GPS receiver for receiving radio waves from GPS satellites is preferably mounted on the roof of the vehicle. Reference numeral 14 represents the main body of the vehicle navigator.
Referring to FIG. 2, if the GPS receiver 3 can capture at least a predetermined number of GPS satellites necessary for calculating the vehicle position, the position determining unit 9 adopts the vehicle position sent from the GPS receiver 3 and outputs it to the controller 12. Conversely, if the GPS receiver 3 cannot capture a predetermined number of GPS satellites necessary for calculating the vehicle position, the relative position calculator 6 calculates the vehicle position on the basis of data output from the acceleration sensor 1 and the gyro sensor 2. The calculation of the vehicle position by the relative position calculator 6 will now be described.
First, the running distance calculator 4 double integrates in the time domain the data output from the acceleration sensor 1 at a predetermined time interval .DELTA.t. Namely, the following two equations (1) and (2) are used to calculate a vehicle velocity V.sub.n and a running distance .DELTA.D.sub.n : ##EQU1## where .DELTA.t is a time interval during which output data is obtained from the acceleration sensor 1, a.sub.i is the acceleration of the vehicle at time i, V.sub.n is the velocity of the vehicle at time n, .DELTA.D.sub.n is a running distance of the vehicle during the time interval .DELTA.t, and V.sub.0 is the initial velocity of the vehicle.
The running azimuth calculator 5 integrates in the time domain the data output from the gyro sensor 2 at the predetermined time interval .DELTA.t. Namely, the following equation (3) is used to calculate a running azimuth .theta..sub.n of the vehicle: ##EQU2## where .DELTA.t is an interval during which output data is obtained from the gyro sensor 2, .omega..sub.i is the angular velocity at the time i, .theta..sub.n is the running azimuth of the vehicle at the time n, and .theta..sub.0 is the initial azimuth.
FIG. 4 is a conceptual diagram illustrating the position calculation by the relative position calculator 6 of the conventional vehicle navigator. Receiving the vehicle running distance .DELTA.D.sub.n and the vehicle running azimuth .theta..sub.n, the relative position calculator 6 performs a cumulative calculation starting from the vehicle initial position by using the following equations (4) and (5) to obtain the vehicle position: ##EQU3## where (X.sub.n, Y.sub.n) are the coordinate values of the vehicle position at time n and (X.sub.0, Y.sub.0) are the coordinate values of the vehicle at the initial position.
Vehicle position information output from the relative position calculator 6 is supplied to the position determining unit 9. The vehicle position information determined by the position determining unit 9 on the basis of the vehicle positions output from the relative position calculator 6 or the GPS receiver 3 is output to the controller 12. Upon reception of the vehicle position information, the controller 12 reads map data of a predetermined area from the map storage 10 and outputs it to the display unit 11.
In this vehicle navigator for calculating the running distance of the vehicle on the basis of the data output from the acceleration sensor 1, the running distance is calculated by double integrating the data detected at the predetermined time interval and output from the acceleration sensor 1 and by performing the cumulative calculation starting from the initial position. In this calculation, the initial conditions of the vehicle, i.e., initial position and initial velocity are required. A vehicle stop state or the like has been used as such initial conditions. However, cumulative calculation starting from the initial position also accumulates errors contained in the data output from the acceleration sensor 1. Therefore, as the running time increases, the errors increase so that the vehicle position displayed on map data differs greatly from the real vehicle position. A method of preventing an increase of errors by using GPS positioning data will now be described.
As data detected with the acceleration sensor 1 at the predetermined time interval .DELTA.t is output, the running distance calculator 4 uses the equations (1) and (2) and the data output from the acceleration sensor 1 to calculate a vehicle velocity (hereinafter called a "first velocity") and a running distance. The GPS receiver 3 outputs a vehicle velocity (hereinafter called a "second velocity") which is calculated by using the Doppler frequency shift of a signal transmitted from each GPS satellite. The velocity correcting unit 8 replaces the first velocity calculated by the running distance calculator 4 by the second velocity supplied from the GPS receiver 3.
If the precision of the second velocity output from the GPS receiver 3 is poor, the correction effects by the velocity correcting unit 8 are degraded. Therefore, the velocity correcting unit 8 corrects the velocity only when precision information output from the GPS receiver indicates that the precision is high.
The precision of the second velocity output from the GPS receiver 3 is judged by the reliability judging unit 7. The reliability judging unit 7 judges the reliability of the second velocity by using: information called a user equivalent error (UERE) representative of positioning system errors (such as the stability of a timer installed in a GPS satellite, delay in the transmission of a signal from the GPS, satellite orbit preheat, and the like) received by the GPS receiver 3; information called a DOP which is a geometrical deterioration coefficient related to the positional relationship between the GPS receiver 3 and GPS satellites; information related to the intensity of the radio wave transmitted from each GPS satellite, and other pieces of information. The reliability judging unit 7 stores therein preset threshold values of UERE, DOP, radio wave intensity, and other information and compares the signal output from the GPS receiver 3 with predetermined threshold values to thereby judge the reliability of the second velocity from the GPS receiver.
If the reliability judging unit 7 judges that the second velocity of the vehicle output from the GPS receiver 3 is highly reliable, the velocity correcting unit 8 replaces the first velocity supplied from the running distance calculator 4 with the second velocity, and the running distance calculator 4 calculates the vehicle running distance from the second velocity. In the above manner, accumulation of errors contained in the output data of the acceleration sensor 1 has been reduced conventionally.
As mentioned above, in the vehicle navigator for calculating the running distance of the vehicle on the basis of the data output from the acceleration sensor at the predetermined time interval and performing cumulative calculation starting from the initial position, as the running distance from the initial position becomes long, errors are accumulated so that the precision of the calculated running distance becomes low. In order to avoid the accumulation of errors, a method has been proposed in which output data of the acceleration sensor is corrected by GPS positioning data. However, if the radio waves from GPS satellites are blocked for a long time, such as while a vehicle is running in a long tunnel, the output data of the acceleration sensor cannot be corrected, errors are accumulated, and the precision of the calculated running distance becomes very poor.